Self-imaging waveguide optical polarization or wavelength splitters

ABSTRACT

Two new techniques for the separation of orthogonally polarized light or  ht of 2 arbitrary wavelengths by use of a waveguide optical device, based on either 1) simultaneous crossed and barred 1-by-1 off-center self-imaging, or 2) out-of-phase self-imaging. Simultaneous crossed and barred 1-by-1 off-center self-imaging utilizes a waveguide optical polarization splitter having an input waveguide containing TE and TM, a multimode interference region, aligned so that TE and TM refractive indices are very different, and with the length set so that the polarization with the lower refractive index is bar self-imaged while the other polarization is cross self-imaged, an output waveguide containing polarization with higher refractive index, and another output waveguide containing the other polarization. Both of these methods may also be used to separate light of 2 arbitrary wavelengths. Out-of-phase self-imaging utilizes a waveguide optical polarization splitter having an input waveguide containing TE and TM, a 1-by-2 polarization-independent self-imaging power splitter, an intermediate waveguide of length L par  containing TE and TM, an intermediate waveguide of length L par  +2*L perp  containing TE and TM, a 2-by-2 self-imaging coupler, and output waveguide containing TE only, and an output waveguide containing TM only.

BACKGROUND OF THE INVENTION

This invention pertains to the field of integrated optics and photonics.Applications include optical and electronic communications, antennaremoting, cable television, waveguide sensing, and control of phasedarray antennae.

Integrated-optic devices are made according to photolithographic andmicrofabrication techniques. This makes possible mass production, in thesame way as for electrical integrated circuits. The most commonelectro-optic substrate materials for integrated-optic devices are thesemiconductors gallium arsenide (GaAs) and indium phosphide (InP) andlithium niobate, a ferroelectric insulating crystal. Lithium niobate isa strong, easily polished nonhydroscopic crystal, with a goodelectro-optic coefficient. It also has low optical transmission loss.

The emerging field of integrated optical systems has generated a numberof components analogous to those employed in electronic circuits. Forexample, there are devices for performing beam-splitting and/orrecombination functions such as those shown in U.S. Pat. No. 5,410,625.There are devices for performing optical mixing, such as those shown inU.S. Pat. No. 5,475,776. And there are devices for performing signalrouting, such as those shown in U.S. Pat. No. 5,428,698.

There are presently a number of waveguide techniques for the separationof orthogonally polarized light. One technique is power splittingfollowed by TE/TM mode filtering, which achieves good extinction ratios,but sacrifices half of the power. In addition, multiple fabricationsteps are required. Another technique uses grating devices, but thesedevices do not achieve sufficient extinction ratios; the ratios are onlyon the order of 9-11 dB, compared to a desired 20 dB or more. A thirdtechnique utilizes directional couplers and Mach-Zehnderinterferometers, which work well in theory, but have very tighttolerances. Good extinction ratios are very difficult to achieverepeatably, and difficult to maintain over wide temperature ranges, dueto thermal expansion/contraction. In consequence, these types of devicesgenerally require active compensation. Mach-Zehnder interferometers alsointroduce an undesirable phase difference between the TE and TM outputs.Finally, asymmetric y-branch devices and overlapping self-imagingdevices work well, but require multiple fabrication steps and only workon certain materials. Generally, for the currently available techniqueseither the performance is inadequate or the fabrication is complicatedand/or material restricted. The situation is similar for the separationof wavelengths.

BRIEF SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide three newtechniques for the separation of orthogonally polarized light or lightof 2 arbitrary wavelengths by use of a waveguide optical device.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in this art from the detaileddescription, wherein only the preferred embodiment of the presentinvention is shown and described, simply by way of illustration of thebest mode contemplated of carrying out the present invention. As will berealized, the present invention is capable of other and differentembodiments, and its several details are capable of modifications invarious obvious respects, all without departing from the presentinvention. Accordingly, the drawings and descriptions are to be regardedas illustrative in nature, and not as restrictive.

These and other objects are achieved by providing three new techniquesfor the separation of orthogonally polarized light or light of 2arbitrary wavelengths by use of a waveguide optical device, based oneither 1) simultaneous 1-by-1 and 1-by-2 self-imaging, or 2)simultaneous crossed and barred 1-by-1 off-center self-imaging, or 3)out-of-phase self-imaging. Embodiment #1 (simultaneous 1-by-1 and 1-by-2self-imaging) utilizes a waveguide optical polarization splitter havingan input waveguide containing TE and TM polarized light, a multimodeinterference region, aligned so that the TE and TM refractive indicesare very different, and with the length set so that the polarizationwith the lower refractive index is singly self-imaged while the otherpolarization is doubly self-imaged, an output waveguide containing thelight polarized with higher refractive index, and two additional outputwaveguides each containing half the power of the waveguide containingthe light polarized with the higher refractive index. The samearchitecture can be used for wavelength separation/combination.Embodiment #2 (simultaneous crossed and barred 1-by-1 off-centerself-imaging) utilizes a waveguide optical polarization splitter havingan input waveguide containing TE and TM polarized light, a multimodeinterference region, aligned so that the TE and TM refractive indicesare very different, and with the length set so that the light polarizedwith the lower refractive index is bar self-imaged while the light withthe other polarization is cross self-imaged, an output waveguidecontaining light polarized with a higher refractive index, and anotheroutput waveguide containing light with the other polarization. The samearchitecture can be used for wavelength separation/combination.Embodiment #3 (out-of-phase self-imaging) utilizes a waveguide opticalpolarization splitter having an input waveguide containing TE and TMpolarized light, a 1-by-2 polarization-independent self-imaging powersplitter, an intermediate waveguide of length L_(par) containing TE andTM polarized light, an intermediate waveguide of length L_(par)+2*L_(perp) containing TE and TM polarized light, a 2-by-2 self-imagingcoupler, and output waveguide containing TE only polarized light, and anoutput waveguide containing TM only polarized light.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a top view of the field evolution through the multimodeinterference region of a 1-by-1 self-imaging device for p=1, with thebrightness at any point proportional to the amplitude of the electricfield at that point. This plot was generated from data produced by acomputer program which models multimode interference devices.

FIG. 2 is a top view of the field evolution through the multimodeinterference region of a 1-by-2 self-imaging device for q=1, with thebrightness at any point proportional to the amplitude of the electricfield at that point. This plot was generated from data produced by acomputer program which models multimode interference devices.

FIG. 3 is a schematic top view of embodiment #1 of my invention, asimultaneous 1-by-1 and 1-by-2 self-imaging waveguide opticalpolarization splitter in which, in general, p and q1.

FIG. 4 is a top view of the field evolution through the multimodeinterference region of a barred 1-by-1 off-center self-imaging devicefor p=1, with the brightness at any point proportional to the amplitudeof the electric field at that point. This plot was generated from dataproduced by a computer program which models multimode interferencedevices.

FIG. 5 is a top view of the field evolution through the multimodeinterference region of a crossed 1-by-1 off-center self-imaging device,for q=1, with the brightness at any point proportional to the amplitudeof the electric field at that point. This plot was generated from dataproduced by a computer program which models multimode interferencedevices.

FIG. 6 is a schematic top view of embodiment #2 of my invention, asimultaneous crossed and barred 1-by-1 off-center self-imaging waveguideoptical polarization splitter in which, in general, p and q1.

FIG. 7 is a top view of the field evolution through the multimodeinterference region of a 2-input self-imaging device in which the inputsare in phase, with the brightness at any point proportional to theamplitude of the electric field at that point. This plot was generatedfrom data produced by a computer program which models multimodeinterference devices.

FIG. 8 is a top view of the field evolution through the multimodeinterference region of a 2-by-2 self-imaging device in which the inputsare 90 degrees out of phase, with the brightness at any pointproportional to the amplitude of the electric field at that point. Thisplot was generated from data produced by a computer program which modelsmultimode interference devices.

FIG. 9 is a schematic top view of embodiment #3 of my invention, anout-of-phase self-imaging waveguide optical polarization splitter.

DETAILED DESCRIPTION OF THE INVENTION

Waveguide polarization or wavelength splitters are required for thefollowing applications:

a) Polarization or wavelength diversity receivers for coherentcommunication;

b) Optical fiber sensing of electric or magnetic fields using theFaraday effect;

c) Optical power doubling.

The use of a waveguide optical device for the separation of light withorthogonal polarizations or arbitrary wavelengths will be extremelyadvantageous in the field of optical control and communications and willyield several advantages over currently available waveguide separationdevices. The advantages of my inventive techniques are:

(a) Ease of Manufacture; devices can be manufactured in one step with nocladding and with reasonable tolerances.

(b) Good Performance; devices will have low loss and low polarizationcrosstalk.

(c) Environmental Insensitivity; devices will be insensitive to externalenvironments.

(d) Small Size.

(e) Passive; No electric fields are required and there is no powerconsumption.

(f) Generally Implementable; any birefringent material for embodiments#1 or #2, or any material for embodiment #3, and most fabricationprocesses for embodiments #1, #2, and #3.

(g) Simplicity; especially so for embodiments #1 and #2.

EMBODIMENT #1

Embodiment #1, a simultaneous 1-by-1 and 1-by-2 Self-Imaging WaveguideOptical Polarization Splitter 10, is shown schematically in FIG. 3. Thisintegrated optical device consists of an input waveguide 2 containing TEand TM polarized light; a multimode interference region 4 aligned sothat the refractive index for the TE polarized light is very differentfrom the refractive index for the TM polarized light, and with thelength L₁ of region 4 set so that the light polarization modeexperiencing the lower refractive index in region 4 is singlyself-imaged while the light having the other polarization mode is doublyself-imaged; an output waveguide 6 containing the light polarizationmode experiencing the higher refractive index, and two output waveguides8 and 9 each containing half the power of waveguide 6.

The operation of splitter 10 can be explained as follows: the lengthL_(p) of a center-fed 1-by-1 self-imaging device 1, as shown in FIG. 1,is given approximately by: ##EQU1## where n₁ is the effective index ofrefraction of the multimode interference (MMI) region 3 for a particularpolarization mode, W_(eff) is the effective width of MMI region 3, λ isthe wavelength (in vacuum) of the transmitted light, and p is anyinteger greater than zero. Similarly, the length L_(q) of a center-fed1-by-2 self-imaging device 5, shown in FIG. 2, is given approximatelyby: ##EQU2## where n₂ is the effective index of refraction of multimodeinterference region 7 for light having the opposite polarization to thatof device 1 and q is any integer greater than zero. Careful computermodeling of self-imaging devices 1 and 5 shows that they can be designedto have a significant depth-of-focus; i.e., good 1-by-N imaging can beobtained over a range of lengths. Using this knowledge, if n₂ is not toodifferent from n₁, then it is possible to find a p and q such that L_(p)and L_(q) are equal to within their focal depths. This is the key toembodiment #1, as well as embodiment #2 (discussed below). TheSelf-Imaging Waveguide Optical Polarization Splitter 10 shown in FIG. 3can thus be fabricated to act as a 1-by-1 self-imager for light havingone polarization mode (TE or TM) and simultaneously as a 1-by-2 splitterfor light having the other polarization mode. Note that only a singlefabrication step is required, and that this technique is unrestricted.Performance for device 10 is also excellent; calculations for anunoptimized design for 1 μm light in y-cut lithium niobate show a lengthof about 9.5 mm, with crosstalk better than--21 dB and loss less than0.4 dB. This is comparable to the best performance achieved by any typeof waveguide polarization splitter. Length errors of up to 10 μm do notappreciably affect performance. One drawback, however, is that if n₂ isonly very slightly different from n₁, then L becomes very long, andperformance degrades as well. This design method for polarizationseparation is therefore practically restricted to birefringent materialssuch as lithium niobate, silica on silicon, or indium phosphide. Notethat this device can be used in reverse to combine polarizations.

The architecture of embodiment #1 can also be used to separate orcombine wavelengths. The reasoning, explanation and performance aresimilar to those just given except that the restriction to birefringentmaterials does not apply. Of particular importance is that thearchitecture of embodiment #1 can be used to separate/combine thecommonly-used communications wavelengths of 1.3 μm and 1.55 μm.

EMBODIMENT #2

Embodiment #2, a simultaneous crossed and barred 1-by-1 off-centerSelf-Imaging Waveguide Optical Polarization Splitter 20 consists of aninput waveguide 22 containing TE and TM polarized light; multimodeinterference region 24 aligned so that the refractive index for the TEpolarized light is very different from the refractive index for the TMpolarized light, and with the length L₂ set so that the lightpolarization mode experiencing the lower refractive index is barself-imaged while the other light polarization mode is crossself-imaged; an output waveguide 26 containing the light polarizationmode experiencing the higher refractive index, and an output waveguide28 containing the other light polarization mode.

The operation of device 20 can be explained as follows: a 1-by-1self-imaging device 12 with an off-center input and barred output (i.e.,offset to the same side of MMI region 13 as the input) in shown in FIG.4. The length L_(p) of region 13 is given approximately by: ##EQU3##where the symbols have the same meaning as in embodiment #1. Similarly,a 1-by-1 self-imaging device 14 with an off-center input and crossedoutput (i.e., offset to the opposite side of MMI region 15 as the input)is shown in FIG. 5. The length L_(q) of region 15 is given approximatelyby: ##EQU4## As in embodiment #1, if n₂ is not too different from n₁,then it is possible to find a p and q such that L_(p) and L_(q) areequal to within their focal depths. One thus obtains a Self-ImagingWaveguide Optical Polarization Splitter 20, as shown in FIG. 6, whichacts as a barred 1-by-1 self-imager for light having one polarizationmode (TE or TM) and simultaneously as a crossed 1-by-1 self-imager forlight having the other polarization mode. Any fabrication method may beused, however, the practical restriction to birefringent materials stillapplies. Note that this device can be used in reverse to combinepolarizations.

The architecture of embodiment #2 can also be used to separate orcombine wavelengths. The reasoning, explanation and performance aresimilar to those just given except that the restriction to birefringentmaterials does not apply. Of particular importance is that thearchitecture of embodiment #2 can be used to separate/combine thecommonly-used communications wavelengths of 1.3 μm and 1.55 μm.

EMBODIMENT #3

Embodiment #3, shown in FIG. 9, is an out-of-phase Self-ImagingWaveguide Optical Polarization Splitter 30, and consists of inputwaveguide 32 containing TE and TM polarized light, a 1-by-2polarization-independent self-imaging power splitter 34, an intermediatewaveguide 35 of length L_(par) containing TE and TM polarized light, anintermediate waveguide 37 of length L_(par) +2*L_(perp) containing TEand TM polarized light, a 2-by-2 self-imaging coupler 36, an outputwaveguide 38 containing TE polarized light only, and an output waveguide40 containing TM polarized light only.

Embodiment #3 begins by splitting the TE and TM polarized light in inputwaveguide 32 into two equal parts using a 1-by-2 self-imaging splitter34, such as the one shown in FIG. 2, but designed to be polarizationindependent. The two parts then propagate through intermediatewaveguides 35 and 37 and are then fed into a 2-by-2 self-imaging coupler36.

If the two parts were to arrive in phase at the input of coupler 36,then 2-by-2 self-imaging would occur, which would accomplish nothing, asshown in FIG. 7. Instead, intermediate waveguide 37 is made longer thanintermediate waveguide 35 so that the two parts arrive with a phasedifference of (2p-1)π/2. This results in off-center 2-by-1 self-imaging,as shown in FIG. 8, for which the parity of p determines the outputwaveguide. Polarization splitting is accomplished by using thebirefringence of the intermediate waveguides 35 and 37 so that p is evenfor one polarization mode and odd for the other.

FIG. 9 schematically depicts an idealized architecture for embodiment#3; an actual device would have gradual curves in intermediate waveguide37, not right angles as shown. In the case of a birefringent substrate,the refractive index is n₁ for TE modes and for TM modes propagatingalong the main axis of the device. The refractive index is n₂ for TMmodes propagating perpendicular to the main axis of the device. Then atthe inputs to 2-by-2 coupler 36, the phase differences will be:

Δφ_(TE) =4πn₁ L_(perp) /λ

Δφ_(TM) =4πn₂ L_(perp) /λ

and for polarization splitting we require:

Δφ_(TE) =(4p+1)π/2

Δφ_(TM) =(4q+3)π/2

Combining these four equations yields the requirement that integers pand q be found such that: ##EQU5## to a good approximation. The resultcan be used to find L_(perp) using: ##EQU6## Note that there is norestriction on L_(par). As an example, for y-cut lithium niobate a 1 μm,L_(par) obtains a value of 2.7 μm. An unoptimized design using a 40 μmwide 2-by-2 coupler show a length of about 12 mm, with crosstalk betterthan -32 dB and a loss of 0.5 dB. Narrower couplers give lower loss andshorter overall length but higher crosstalk, and vice versa for widercouplers.

A key difference between embodiment #3 and embodiments #1 and #2 is thatembodiment #3 is more able to exploit the weak birefringence whichoccurs even for isotropic substrates, such as z-cut lithium niobate,gallium arsenide, glass, and polymer, due to the difference in boundaryconditions for TE and TM modes. If the refractive index is n₁ for TEmodes and n₂ for TM modes, regardless of propagation direction, then oneagain derives the equations above. L_(perp) is longer but stillreasonable. An added bonus is that the tolerances are also increased.The extraordinary performance numbers given above for y-cut lithiumniobate still apply for z-cut lithium niobate, and similar performancecan be obtained for gallium arsenide, glass, etc. As in embodiments #1and #2, any fabrication method may be used. Note that this device can beused in reverse to combine polarizations.

It will be readily seen by one of ordinary skill in the art that thepresent invention fulfills all of the objects set forth above. Afterreading the foregoing specification, one of ordinary skill will be ableto effect various changes, substitutions of equivalents and variousother aspects of the present invention as broadly disclosed herein. Itis therefore intended that the protection granted hereon be limited onlyby the definition contained in the appended claims and equivalentsthereof

Having thus shown and described what is at present considered to be thepreferred embodiment of the present invention, it should be noted thatthe same has been made by way of illustration and not limitation.Accordingly, all modifications, alterations and changes coming withinthe spirit and scope of the present invention are herein meant to beincluded.

I claim:
 1. A simultaneous 1-by-1 and 1-by-2 self-imaging waveguideintegrated optical polarization splitter comprising:an input waveguidecontaining orthogonally polarized light of two modes; a multimodeinterference region having a refractive index, the multimodeinterference region aligned with said input waveguide such that therefractive index of the multimode interference region for saidorthogonally polarized light is different for said two modes, with thelength of said multimode interference region set such that the lightpolarization mode experiencing the lower refractive index in said regionis singly self-imaged while the light having the other polarization modeis doubly self-imaged; a first output waveguide containing the lightpolarization mode experiencing the higher refractive index; and secondand third output waveguides each containing half the power of the otherlight polarization mode.
 2. The integrated optical polarization splitterof claim 1 wherein said two orthogonally polarized light modes are modesTE and TM.
 3. The integrated optical polarization splitter of claim 2wherein the multimode interference region of said splitter is made froma strongly uniaxial or biaxial material.
 4. The integrated opticalpolarization splitter of claim 3 wherein said uniaxial or biaxialmaterial is lithium niobate.
 5. The integrated optical polarizationsplitter of claim 3 wherein said uniaxial or biaxial material is silicaon silicon.
 6. The integrated optical polarization splitter of claim 3wherein said uniaxial or biaxial material is indium phosphide.
 7. Asimultaneous crossed and barred 1-by-1 off-center self-imaging waveguideintegrated optical polarization splitter comprising:an input waveguidecontaining orthogonally polarized light of two modes; a multimodeinterference region having a refractive index, the multimodeinterference region aligned with said input waveguide such that therefractive index of the multimode interference region for saidorthogonally polarized light is different for said two modes, with thelength of said multimode interference region set such that the lightpolarization mode experiencing the lower refractive index in said regionis bar self-imaged while the light having the other polarization mode iscross self-imaged; a first output waveguide containing the lightpolarization mode experiencing the higher refractive index in saidregion; and a second output waveguide containing the other lightpolarization mode.
 8. The integrated optical polarization splitter ofclaim 7 wherein said two orthogonally polarized light modes are modes TEand TM.
 9. The integrated optical polarization splitter of claim 8wherein the multimode interference region of said splitter is made froma strongly uniaxial or biaxial material.
 10. The integrated opticalpolarization splitter of claim 9 wherein said uniaxial or biaxialmaterial is lithium niobate.
 11. The integrated optical polarizationsplitter of claim 9 wherein said uniaxial or biaxial material is silicaon silicon.
 12. The integrated optical polarization splitter of claim 9wherein said uniaxial or biaxial material is indium phosphide.
 13. Anout-of-phase self-imaging waveguide integrated optical polarizationsplitter comprising:an input waveguide containing orthogonal polarizedlight of two modes; a 1-by-2 polarization-independent self-imaging powersplitter; a first intermediate waveguide containing said two modepolarized light; a second intermediate waveguide longer than said firstintermediate waveguide containing said two mode polarized light; a2-by-2 self-imaging coupler having said first and said secondintermediate waveguides as inputs; a first output waveguide containingone mode polarized light only; and a second output waveguide containingthe other mode polarized light only.
 14. The integrated opticalpolarization splitter of claim 9 wherein said two orthogonally polarizedlight modes are modes TE and TM.